1. Field of the Invention
The present invention relates to a process for improving the cetane number or index of the directly distilled diesel fuels.
2. Description of the Prior Art
The diesel fuels of direct distillation, or direct diesel fuels or diesel fuels proper, are median distillates, intermediate between light distillates, gas and gasolines and aviation fuels on the one hand, and the heavy residues on the other, produced by distillation at atmospheric pressure, at which the refining of crude oils begins. The composition and properties of the direct diesel fuels differentiate them in known manner from viscoreduction or catalytic cracking diesel fuels produced after distillation, under vacuum, of the aforementioned heavy residues. The distillation range under atmospheric pressure of a direct diesel fuel most typically ranges from about 150.degree. C. -200.degree. C. to about 350.degree. C.-450.degree. C.
The direct diesel fuels are essentially intended for use as fuels for diesel engines and as domestic heating fuels.
They must have a sufficiently high compression ignition rating, expressed by their cetane number, which is comparable to the octane number rating for gasoline.
The desirability of having available to industry such diesel fuels having a high cetane number has been recently described, for example in the European patent published under No. 0,252,606. Presently, cetane numbers higher than about 45, preferably higher than 50, are considered desirable.
One proposed solution to ensure a satisfactory cetane number for direct diesel fuels entails incorporating an additive therein. The disadvantage of such a technique is that the selection of the additive is restricted to one that is simultaneously relatively insoluble in water and chemically stable relative thereto, sufficiently soluble in the diesel fuel under the preferred conditions of use, stable in diesel fuel at all storage temperatures, and, finally, noncorrosive and nontoxic.
For a definition of the terms employed herein, such as motor diesel fuel, cetane number, etc., reference may be made to the following tests: Le langage petrolier (The language of petroleum), Gauthier-Villars, Paris (1964); Le Petrole, raffinage et genie chimique (Petroleum, refining and chemical engineering), edited by Pierre Wuithier, 2nd edition, Technip Publisher, pp. 16, 17, 29, 30 (1972), or the article by D. Indritz, "Preprints, Symposium of the Chemistry of Cetane Number Improvement", Miami Beach, Apr. 28-May 3, 1985, pp. 282-286.
The direct diesel fuels having an improved cetane index may be marketed in and of themselves or they may be admixed to increase the cetane index of diesel fuels having a lower cetane index.